An uplink (UL) bearer split is introduced in the Release 13 version of the Long Term Evolution (LTE) standard. In a UL bearer split, a user equipment (UE) is may send its packet data convergence protocol (PDCP) data packets through two different link chains consisting of radio link control (RLC), Medium access control (MAC) and physical layer (PHY) entities, thereby enhancing the UL data rates. The UE receives separate UL allocations or grants on these two UL links and accordingly builds a protocol data unit (PDU) and submits to lower layers for transmission.
The PDCP entities are located in a PDCP sublayer. Several PDCP entities may be defined for the UE. Each PDCP entity carrying the user plane data may be configured to use header compression. Each PDCP entity is carrying the data of one radio bearer. Robust header compression (ROHC) protocol is supported by the PDCP. Every PDCP entity may use at least one ROHC compressor instance and at least one ROHC decompressor instance. The PDCP entity is associated with a control plane or the user plane, depending on which radio bearer it is carrying data for.
The PDCP provides services to a radio resource control (RRC) and user plane upper layers at the UE or to a relay at an evolved node B (eNB). For example, services provided by PDCP to the upper layers may include transfer of user plane data, transfer of control plane data, header compression, ciphering, and integrity protection.
Apart from upper layer data, a PDCP entity also generates a control PDU on its own, which is commonly referred to as a “PDCP control PDU”. For example, the PDCP Control PDU is used to convey a PDCP status report, following a PDCP re-establishment, indicating which PDCP service data units (SDUs) are missing and are not header compression control information, e.g., interspersed ROHC feedback.
Buffer status reporting (BSR) for PDCP data packets is transmitted to a master eNB (MeNB) and a secondary eNB (SeNB) based on a threshold approach thereof. The BSR report includes an amount of data buffered for PDCP data packets and is conveyed to an eNB, i.e., either the MeNB or the SeNB, which is preconfigured by network when PDCP data is less than threshold. However, when PDCP data is above the threshold, the BSR is triggered to both eNBs, which is commonly referred to as “a double reporting approach”.
However, there are potential issues in the UL split bearer operation for PDCP control PDU transmission.
For example, in dual connectivity (DC) for the UL split bearer operation, there are two possible links to the MeNB and the SeNB, respectively. The BSR and grant allocations on these two links will determine how the PDCP packets are transmitted on these links.
Also, the connection between the SeNB to the MeNB is through a non-ideal backhaul. Therefore, PDCP packets sent to the SeNB are forwarded to the MeNB (where peer PDCP entity resides) through this non-ideal backhaul and experience transfer delay. In this situation, transmission of a PDCP control PDU also suffers.
PDCP Control PDU is critical and any delay in its reception will in turn lead to further delay in processing, transmission, and retransmission of the PDCP packets. For example, a PDCP control PDU may carry status information on the reception status of the packets in order to inform about which packets should be retransmitted. Therefore, if a PDCP control PDU is treated as other data PDUs, delay issues, e.g., a scheduling delay, a backhaul transfer delay, a handover intermission/delay, a discontinuous reception (DRX) wakeup delay, etc., because of a grant allocation or a transfer over the backhaul cannot be avoided.
Thus, there is a need of a simple and robust mechanism for handling the aforementioned issues and reducing delay involved in processing, transmitting, and retransmitting PDCP packets (i.e., PDCP control PDU transmission), thereby increasing the throughput of UE configured with DC.